This laboratory has been testing the hypothesis that toxic exposure may be identified, and the level of internal exposure determined, by the chemical detection, characterization and quantitation of covalent hemoglobin adducts. The quantitation of hemoglobin adducts offers several advantages over other proteins, including ease of isolation, limited catabolism, and prolonged life-time. Therefore, the detection and quantitation of hemoglobin adducts appears to be an excellent biomarker of toxic exposure. Preliminary studies from this laboratory have identified the accumulation of adducts during subacute exposures. The amount of the adducts is proportional to both the length of exposure and the level of exposure. Thus, it appears that the amount of hemoglobin adducts is a useful indicator of cumulative exposure. In addition, two avenues have been explored to distinguish between acute and chronic exposures. First, the elimination of adducts following an acute (single) exposure follows linear kinetics with a life-time approximately equal to that of the red cell; in contrast, the elimination of adducts following chronic exposures is more rapid and nonlinear. Secondly, when erythrocytes are fractionated into subpopulations of differing ages, a larger amount of adduct is present in older cell populations following chronic exposure, while younger cell populations are equally exposed in the acute setting. These findings suggest that, while the level of hemoglobin adducts reflects the cumulative exposure, the elimination kinetics and distribution pattern among red cell subpopulations may help distinguish the pattern of exposure, whether acute or chronic. We propose to extend these observations by studying four chemically-unrelated toxicants to explore the relationships between internal exposure and hemoglobin adducts, including the kinetics of accumulation, the kinetics of elimination, and the distribution of adducts among erythrocyte subpopulations. These studies will attempt to validate hemoglobin dosimetry as a useful biomarker of internal exposure, a biomarker with great implications for human monitoring and risk assessment.